Commonly assigned U.S. Pat. No. 5,159,513 discloses a magnetoresistive (MR) sensor that utilizes the spin valve effect. The sensor comprises a rectangular multi-layered structure deposited on a glass or other suitable substrate. The multi-layered structure includes a "free" layer of soft magnetic material and a "pinned" layer in which the magnetization is fixed in a direction parallel to the width of the sensor by use of a hard magnetic material as the first layer or, if preferred, by use of an antiferromagnetic layer to pin the first layer by exchange coupling. The free and pinned layers are separated by a thin nonmagnetic metallic spacer layer, such as of copper. The magnetization of the free layer is normally in a direction along the length of the sensor, but is free to rotate therefrom in a positive or negative transition direction through an angle determined by the magnitude of the magnetic field being sensed.
In a spin valve structure, the change in resistance is proportional to the change in the angle between the directions of the magnetization in the two magnetic layers (i.e., the free layer and the pinned layer). The change in resistance of this multi-layered structure relative to a constant background value is then proportional to sin .theta. averaged over the height of the sensor, where as heretofore indicated, .theta. is the angle of the magnetization in the free layer with respect to the longitudinal axis of the sensor. Because the resistance change is proportional to sin .theta., the signal produced by the sensor will be linear throughout the applied field for small field values. However, the sensor output becomes nonlinear when the free layer is magnetically saturated during a positive or negative excursion of the applied field and thus limits the peak-to-peak signal that can be produced by the sensor.
The ideal quiescent magnetic state for the sensor is achieved when .theta.=0 over the entire height of the free layer. This ideal quiescent state is that which is furthest from magnetic saturation under both positive and negative field excitations. It generates an ideal bias profile that maximizes the linear dynamic range to provide a larger signal output and/or improved linearity. However, this ideal bias profile cannot be achieved with spin valve structures disclosed in this cited patent or any other prior art known to applicants. This inability to obtain the ideal bias profile is due mainly to the flux coupling between the pinned layer and the free layer and by the field produced by the current that flows through and normal to the various layers of the spin valve structure. Attempts to balance out these undesirable effects by adjusting the current direction and placement of the spin valve structure in the gap resulted in a very nonuniform bias profile.
There is a need for a magnetic disk storage device having an MR sensor element with a spin valve structure in which the ferromagnetic exchange coupling and the field produced by the current are constant across the entire MR element so that their effects can be appropriately cancelled.